1. Field of the Invention
The present invention relates to an active matrix type organic EL drive circuit and an organic EL display device and, in particular, the present invention relates to an active matrix type organic EL display device of a portable telephone set or a PHS, etc., which is capable of performing an initial charging (charging at light emission start time for emitting light earlier) of organic EL elements of an active matrix type organic EL panel, reducing a time required to write a drive current value to a capacitor of a pixel circuit and improving luminance of the organic EL elements and which is suitable for a high luminance color display.
2. Description of the Prior Art
It has been known that an organic EL display device, which realizes a high luminance display by spontaneous light emission, is suitable for a display on a small display screen and the organic EL display device has been attracting public attention as the next generation display device to be mounted on a portable telephone set, a PHS, a DVD player or a PDA (Personal Digital Assistants), etc. Known problems of such organic EL display device are that, since, when it is driven by voltage as in a liquid crystal display device, luminance variation thereof becomes substantial and that, since there is difference in sensitivity of organic EL element between R (red), G (green) and B (blue), a control of luminance of a color display becomes difficult.
In view of these problems, an organic EL display device using current drive circuits has been proposed recently. For example, JPH10-112391A discloses a technique in which the luminance variation problem is solved by employing a current drive system.
An organic EL display panel of an organic EL display device for a portable telephone set, a PHS, etc., having 396(=132×3) terminal pins for column lines and 162 terminal pins for row lines has been proposed. However, there is a tendency that the number of column lines as well as row lines is further increased.
An output stage of a current drive circuit of such organic EL display panel of either the active matrix type or the passive matrix type includes a current source drive circuit, such as an output circuit constructed with a current mirror circuit, for each of the terminal pins.
In the active matrix type organic EL display panel, a pixel circuit composed of a capacitor and a transistor is provided for each display cell (pixel). An organic EL element (referred to as “OEL element”, hereinafter) is current-driven through the transistor, which is driven correspondingly to a voltage stored in the capacitor. The drive system is either a digital drive system in which the drive current of the OEL element is binary-controlled and an analog control in which the drive current is controlled by an analog input data. In the case of the digital drive system, a display area is controlled by providing a sub pixel in the pixel or the tone of the display element is controlled according to a drive time by a time-division of a light emitting time. In the case of the analog drive system, there are a voltage setting type (voltage program system) and a current setting type (current program system). In the voltage setting type system, a terminal voltage of the capacitor of each pixel circuit is set according to a voltage signal and, in the current setting type system, the terminal voltage of the capacitor is set according to a current signal.
In such active matrix type OEL panel, an unevenness of luminance tends to occur due to variation of an operating threshold value of the drive transistor of each pixel circuit. Since it is difficult to unify the operating threshold value of the drive transistor of each display element in a manufacturing process, it has been considered to restrict the luminance variation by controlling the voltage of the capacitor of each pixel circuit. In order to realize this, a threshold compensation circuit is provided in the pixel circuit. As an example of the compensation circuit, there are a circuit of the above mention voltage program system and a circuit of the current program system.
The voltage program system uses four transistors and two capacitors in each pixel circuit and includes two lines for compensating for variation of the operating threshold value of the drive transistor in addition to a data line and a selection line. A current drive, which is not influenced by the threshold value of the drive transistor is performed by charging the two capacitors with a predetermined timing by applying a control signal to the two lines of the voltage program system.
The current program system is constructed with three transistors including a drive transistor and a switch transistor for setting a specific voltage. In addition to a data line and two selection lines, a power source line of the specific voltage Vdd. First, the drive transistor is disconnected by the switch transistor to charge the capacitor. Thereafter, the drive transistor is connected to the capacitor by the switch transistor to supply power from the source line to thereby current-drive the OEL element.
Incidentally, a current drive circuit of a passive matrix type organic EL display panel uses current having a peak current, for restricting luminance variation by emitting light earlier by charging the OEL element having a capacitive load characteristics. On the other hand, the active matrix type organic EL display panel temporarily writes a voltage corresponding to a drive current in the capacitor of the pixel circuit and then generates the drive current corresponding to the written voltage of the capacitor. Therefore, the OEL element of the active matrix type organic EL panel is not driven by a peak current. As a result, it is impossible to emit light earlier as in the passive matrix type organic EL panel and the write time for writing the drive current is necessary. Therefore, it has a problem that a light emitting period is reduced.
The writing of the drive current is usually performed by charging the capacitor of the pixel circuit, which is usually several hundreds pF, with a current of about 0.1 μA to 10 μA and the time required to write the drive current is as long as about 10% or more of the scan period. The light emitting time is reduced correspondingly, resulting in reduction of display luminance. Particularly, when the number of display pixels is increased as in VGA, XGA, etc., in which the time control must be performed within a limited time, the previously mentioned defect becomes serious.